1. Field of the Invention
The present invention relates to piconet wireless networks. More particularly, it relates to baseband clock generation for BLUETOOTH™ radio frequency (RF) integrated circuits, and even more particularly to a direct voltage controlled oscillator (VCO) modulation scheme having particular use for the transmission of frequency shift keying (FSK) type data signals.
2. Background of Related Art
Piconets, or small wireless networks, are being formed by more and more devices in many homes and offices. In particular, a popular piconet standard is commonly referred to as a BLUETOOTH piconet. Piconet technology in general, and BLUETOOTH technology in particular, provides peer-to-peer communications over short distances.
The wireless frequency of piconets may be 2.4 GHz as per BLUETOOTH standards, and/or typically have a 20 to 100 foot range. The piconet RF transmitter may operate in common frequencies which do not necessarily require a license from the regulating government authorities, e.g., the Federal Communications Commission (FCC) in the United States. Alternatively, the wireless communication can be accomplished with infrared (IR) transmitters and receivers, but this is less preferable because of the directional and visual problems often associated with IR systems.
A plurality of piconet networks may be interconnected through a scatternet connection, in accordance with BLUETOOTH protocols. BLUETOOTH network technology may be utilized to implement a wireless piconet network connection (including scatternet). The BLUETOOTH standard for wireless piconet networks is well known, and is available from many sources, e.g., from the web site www.bluetooth.com.
According to the BLUETOOTH specification, BLUETOOTH systems typically operate in a range of 2400 to 2483.5 MHz, with multiple RF channels. For instance, in the US, 79 RF channels are defined as f=2402+k MHz, k=0, . . . , 78. This corresponds to 1 MHz channel spacing, with a lower guard band (e.g., 2 MHz) and an upper guard band (e.g., 3.5 MHz).
To receive a radio frequency (RF) signal from another piconet device, the receiving device must lock onto the transmitted frequency. All devices have a local clock oscillator on which a baseband clock signal in an RF section is based.
While ideally both the transmitting device and the receiving device would include identical local clock oscillator sources, this is not the case in the real world. For instance, clock signals jitter and vary somewhat within desired tolerable limits due to environmental conditions such as the temperature of the device, the exact frequency of the particular crystal oscillator in the device, etc. Design standards typically allow some amount of jitter and gain variation. For instance, the current BLUETOOTH™ piconet network standard specifies that the clock jitter (rms value) should not exceed 2 nS and the settling time should be within 250 uS.
The BLUETOOTH standard also requires that the maximum deviation of a transmitted frequency be in the range 140–175 KHz. However, particularly because of the extremely high frequency of the transmission channels (e.g., 2.4 to 2.5 GHz), it's rather difficult to maintain deviations to within this range. This is particularly true since variations in modulation frequency gain (KMOD) introduced by a modulation path is typically 10 to 15%. Also, the temperature dependence of a varactor used in the clock signal frequency synthesis path is also quite significant (approx. 10%). Process variation from device to device, and even from design to device, can lead to even larger variations in modulation gain.
Thus, it is clear that modulation gain (KMOD) varies in any given transmit path, and control of modulation gain (KMOD) has been a difficult task in the art. Given a large amount of bandwidth for any given number of transmit channels, adequate tolerances can be provided on either sides of each defined channel to prevent interference. However, as bandwidth becomes more scarce and demand continues to increase, there is a need for tighter tolerances and improved methods to meet these tighter tolerances. Accordingly, a more tightly maintained accuracy (referred to herein as “calibration”) of frequency modulation gain in a particular transmit path is required to allow increased numbers of channels in any given frequency range.
FIG. 7 shows a conventional RF front end including a reference frequency input to a single loop PLL having direct data injection.
In particular, as shown in FIG. 7, a reference frequency 104 is injected into the input of a single loop PLL 704 and ultimately transmitted by a suitable power amplifier 106. In such a system, direct modulation may be used wherein data 102 is injected into a voltage controlled oscillator at an output of the PLL 704. While such modulation systems are suitable, they nevertheless fall victim to unintended modulation gain in the channel path in the PLL 704.
One conventional “calibration” method of controlling modulated gain (KMOD) includes the use of a look-up table. The look-up table has been used to compensate for channel and temperature variations due to modulated gain (KMOD). However, these look-up tables require the determination of the actual temperature, assuming that the temperature of the transmit channel can be sensed correctly. Moreover, even with look-up table calibration, other factors causing variations in modulation gain (KMOD) (such as process variation effects) remain un-calibrated and un-cancelled.
There is a need for an improved approach to maintain an accurate modulation within allowed tight frequency tolerances in a modulation path of an RF device (particularly a BLUETOOTH piconet device).